TOMBSTONE, AZ - Hundreds of volunteers, some of them armed, are expected to take up positions along the Mexican border Friday and begin patrolling for illegal immigrants _ an exercise some fear could attract racist crackpots and lead to vigilante violence.

Organizers of the Minuteman Project said the civilian volunteers, many of whom were recruited over the Internet, will meet first for a rally in this one-time silver mining town, then fan out across 23 miles of the San Pedro Valley to watch the border for a month and report sightings of illegal activity to Border Patrol agents.

Minuteman field operations director Chris Simcox described the project as "the nation's largest neighborhood watch group" and said one of the goals is to make the public aware of how porous the border is.

Jim Gilchrist, a retired accountant from Aliso Viejo, Calif., who organized the project, said that some volunteers will carry handguns, which is allowed under Arizona law, but are being instructed to avoid confrontation, even if shot at.

Still, law enforcement officials and human rights advocates are worried about the potential for bloodshed.

Critics contend the project may attract anti-immigrant racists and vigilantes looking to confront illegal immigrants. At least one white supremacist group has mentioned the project on its Web site.

"They are domestic terrorists that represent a danger to the country and could promote a major border conflict that will have serious ramifications and consequences," said Armando Navarro, a University of California-Riverside political science professor and coordinator of the National Alliance for Human Rights, made up mostly of Hispanic activists.

Michael Nicley, chief of the U.S. Border Patrol's Tucson sector, said the volunteers are "not the kind of help the Border Patrol is asking for."

Cochise County Sheriff Larry Dever said he fears immigrant smugglers might open fire on the volunteers.

"I wouldn't anticipate that people of that persuasion would act or react any differently to anybody, citizen or law enforcement alike, if they were confronted and felt like their cargo was in jeopardy," he said.

The project's organizers gave assurances the volunteers will be closely monitored. "If it gets to a situation where someone's life is in danger," said David Helppler, Minuteman security coordinator, "I will end the project."

Project organizers said they expect 800 to 1,000 volunteers. How many might actually show is unclear; similar efforts in the past few years flopped. One of them drew only about a half-dozen people.

The Homeland Security Department planned to announce Wednesday that is is assigning more than 500 additional patrol agents to the porous Arizona border to help keep out potential terrorists and illegal immigrants, The Associated Press learned.

About 155 agents will be immediately sent to Arizona, according to department officials who spoke on condition of anonymity.

The 370-mile Arizona border is considered the most vulnerable stretch of the 2,000-mile southern border. Of the 1.1 million illegal immigrants caught by the Border Patrol last year, 51 percent crossed into the country at the Arizona border.

Some people in this town nearly 30 miles north of the Mexican border, best known as the site of the 1881 shootout at the OK Corral, are eagerly awaiting the volunteers' arrival.

Tombstone Mayor Andree De Journett thinks of the volunteers as tourists and said they could boost the local economy.

"I've met five or six of them, they haven't been too bad so far," he said, estimating that 500 extra visitors for a month could mean $10,000 or more spent locally.

Marilynn Slade, Tombstone's city clerk, said the more attention drawn to illegal immigration, the better.

"The vast majority of the people feel that the feds should be dealing more aggressively with the problem," she said. "There's a huge, huge cry down here."